The present invention relates to a method and a device for non-destructive detection of damage in materials or objects.
There are numerous acoustical techniques previously known for determining damage or defects in materials or objects.
In U.S. Pat. No. 5,086,775 a technique is thus described for estimating the spatial distribution of the vibration amplitude of an object. A low frequency vibration source is used to force the object to oscillate. The frequency of the vibration source is suitably chosen such that an eigenmode of the object is excited. Pulsed ultrasound is then transmitted to the object and by analysis of the Doppler shift of signals reflected by the object the vibration amplitude is determined. In this way abnormal regions in the object are detected due to different mechanical properties of these regions from those of normal regions which in its turn influence the vibration pattern. The described technique is primarily intended for detection of hard tumors surrounded by soft tissues.
In WO 9939194 a method and an apparatus for acoustic detection and location of defects in a structure or ice on the structure are described. A vibration signal of low frequency and a probe signal of a higher frequency are then supplied to the structure. In an undamaged structure these two signals propagate independently without any interaction. In a structure containing a defect, e.g. in the form of flaws, or with ice thereon, the vibration varies the contact area in the flaw or between ice and structure, which results in modulation of the high frequency probe signal. In the frequency domain this modulation manifests itself as frequency sidebands with respect to the frequency of the probe signal.
Thus U.S. Pat. No. 5,732,642 discloses a method and system to detect defects in a material wherein waves of known frequency(ies) are mixed at an interaction zone in the material. As a result, at least one of a difference wave and a sum wave are generated in the interaction zone. The difference wave occurs at a difference frequency and the sum wave occurs at a sum frequency. The amplitude of at least one non-linear signal based on the sum and/or difference waves is then measured. The non-linear signal is defined as the amplitude of one of the difference wave and sum wave relative to the product of the amplitude of the surface waves. The amplitude of the non-linear signal is an indication of defects (e.g., dislocation dipole density) in the interaction zone.
U.S. Pat. No. 5,520,052 discloses a method and apparatus for determining material structural integrity by combining laser vibrometry with damping analysis techniques to determine the damping loss factor of a material. The method comprises the steps of vibrating an area to be tested over a known frequency range and measuring vibrational force and velocity as a function of time over the known frequency range. Using known vibrational analysis, a plot of the drive point mobility of the material over the pre-selected frequency range is generated from the vibrational force and velocity measurements. Once computed, the damping loss factor can be compared with a reference sampling loss factor to evaluate the structural integrity of the material.
U.S. Pat. No. 5,214,960 discloses a method and apparatus for detecting defects in an object by vibrating the object in a plurality of positions. While the test object is vibrating, signals indicative of the vibration of the test object are detected and a signal indicative of a natural vibration of the test object is produced, as well as a signal indicative of a defect-induced vibration of the test object is produced. The signal indicative of the natural vibration and the signal indicative of the defect-induced vibration are compared to determine whether there is a defect in the test object.
U.S. Pat. No. 5,528,924 discloses an acoustic tool for analysis of a gaseous substance specifically a refrigerant gas, to determine whether the sample contains significant contaminants. The refrigerant is tested by introducing a vapour sample into a resonant chamber, which is formed to produce two distinct resonances, the resonator having first and second necks connecting first and second volumes. A frequency generator produces a sweep of frequencies in a band and then includes the two resonances and the sweep is applied to a transducer in one of the volumes. Another transducer responsive to vibrations produces an output signal that varies in response to the amplitude of the vibrations in the chamber. A digital circuit responsive to the frequency generator and the second transducer output determine the centre frequencies for the first and second resonances and determines the frequency width of these resonances to determine quality or sharpness factors for the two resonances. Then the centre frequencies and sharpness factors are compared with storage data and a determination as to the nature and extent of contaminants is made.
U.S. Pat. No. 5,425,272 discloses the use of relative resonant frequency shifts to detect cracks. At least two prominent resonant frequencies of an object are sensed and the frequency difference is measured. The ratio of the frequency difference to one of the prominent resonance frequencies is determined and compared to predetermined criteria. A resonance frequency dependent upon dimensions will shift very little while a resonance frequency dependent upon stiffness will shift a relatively large amount when an object has a crack.
U.S. Pat. No. 5,355,731 discloses a method for grading production quantities of spherical objects. A resonant ultrasound spectroscopy (RUS) spectrum is then generated from a spherical object. Sphere parameter values for the spherical object are determined from first components of the RUS spectrum. An asphericity value of the spherical object is determined from second components of the RUS spectrum and the spherical parameter values. The asphericity value is then compared with predetermined values to grade the spherical product.
U.S. Pat. No. 5,284,058 discloses a method for measuring complex shear or Young's modulus of a polymeric material wherein first and second beams of preselected lengths and different thickness are disposed in parallel spaced relationship firmly held at the ends thereof and first and second spaced gripping members are attached along the beams, a specimen of polymeric material is disposed between confronting surfaces of the gripping members, a time varying force is applied to one beam, the time varying displacements of the beams are measured, and the modulus of the polymeric material is calculated from the measurements.
U.S. Pat. No. 5,179,860 and U.S. Pat. No. 5,144,838 disclose a defect detecting method which includes the steps of vibrating the object, picking up the vibration, and detecting that a spectrum of the characteristic vibration of the object to be measured is separated into two portions. The method can also be used to detect cracks by vibrating an object, picking up the vibration, and detecting that an odd order spectrum of the characteristic vibration of the object to be measured is separated into two portions. A non-through defect can be determined in the same way by detecting that an even order spectrum of the characteristic vibration of the object to be measured is separated into two portions.
U.S. Pat. No. 4,944,185 discloses a method for non-destructively inspecting the integrity of a material by tagging the material, applying the material, activating the tagged particles to cause an inherent structural resonance in the tagged material, monitoring and measuring the structural resonance of the material with a probe, and relating the structure resonance of the material to the structural integrity of the material. This technique has particular application to adhesive materials.
U.S. Pat. No. 4,689,993 discloses a method and apparatus for measuring and mapping vibrations wherein one or more local sensors and a measuring means make local registrations and frequency decompositions of the vibrations of an oscillating object. The same sensors and measuring means can also be used with an image-forming unit and associated measuring means for local and image-forming recording of the vibrations of an oscillating object.
Further, in U.S. Pat. No. 3,958,450 a technique is described for determining surface properties, such as hardness, of materials by studying vibrations excited in the surface.
U.S. Pat. No. 5,216,921 describes a method and an apparatus for detecting defects and different hardness portions of an object with protrusions. Mechanical vibrations are then applied to the test object and by spectral analysis of data about how the different protrusions are vibrating defects and/or different hardness portions are determined in each of the protrusions.
U.S. Pat. No. 5,777,891 discloses an ultrasonic technique for real-time detection of flaws. A plurality of ultrasonic impulses are then injected to the material and echoes caused by discontinuities in the material are analysed regarding signal amplitude, travel time and spreads are compared with corresponding values of pattern for known discontinuities in the material for localising flaws. The described technique is primarily intended for use on rails, and in particular for separating true flaws from intervals between rails in rail junctions.
U.S. Pat. No. 6,023,980 finally, deals with the study of fatigue in materials. A test machine is described used for then applying static and dynamic stress loadings with frequencies in the range of 1000-4000 Hz.
A new physical phenomena in solid material, named “slow dynamics”, has recently been discovered, see Robert A. Guyer and Paul A. Johnson, “Non-linear Mesoscopic Elasticity: Evidence for a New Class of Materials”, Physics Today, April 1999, pp. 30-36. Slow dynamics is a transient or temporary change in the elastic modulus of damaged materials, related to non-linearities due to the presence of cracks in such damaged materials.